1. Technical Field
This invention concerns a process for verification of the radiation dose delivered to patients undergoing radiation therapy.
2. Prior Art
Medical linear accelerators and Cobalt 60 external beam units {represented by 22 in FIGS. 1 and 2) are well known machines used to treat human beings for cancer. The radiation source in these machines are generally mounted in a gantry that can rotate around the patient. A target area within the patient may therefore be irradiated from different directions. Before irradiation the treatment is typically planned on a computer using algorithms that can simulate the radiation beams and allow the medical personnel to design the treatment beam positions and field modifiers that may be placed within the treatment beams.
Inherent in any activity carried out, by human beings in particular, is the possibility of errors. These errors may involve a misunderstanding between the physician prescribing the radiation dose and the technician who generally operate, the treatment planning system to develop a plan. Individuals might make mistakes in the generation of the plan, such as setting the wrong distance for a field as one example. The development of the treatment plan might be a complex process involving devices placed in the beam to modify the radiation field 34 in FIG. 1 produced by a linear accelerator 22. The radiation field emerges from a point source at 20. The beam modifiers such as the group represented by 32 in FIG. 1, may include shielding blocks 28, wedges 26, compensating filters 30, and dynamic intensity modulation with multi-leaf collimators represented at 24, to name a few common such devices. Compensating filters 30 are typically individually designed and manufactured for the particular field 34 for a particular patient 42. Multiple fields 34 are typically employed to converge upon a single contiguous treatment volume.
Any error in the weighting of these fields, or in the modifying devices 32 placed in the fields 34, will lead to an error in the final dose the patient receives. We also must consider the nature of the radiation itself. Ionizing radiation cannot be seen, heard, felt, tasted, or smelled and so there is no sensual feedback to the operator of the treatment planning 46 and delivery equipment 22. Radiation can only be measured with complex equipment by measuring the effects of the radiation, namely the ionization produced in air and other effects. Further, the delivery of the treatment requires that the correct devices be placed in the beam in the intended correct position and that their effects are properly accounted for by the treatment planning system 46. This lengthy and complex process has multiple opportunities for errors to be committed by persons or machines.
Most methods currently employed for preventing mistakes involve a review of the plan, the treatment beam setup, and the patient's position. However detailed information about the dose actually delivered to the patient is hard to come by, as it is difficult to make measurements within the patient's body. The standard procedures for quality control generally call for the checking of each component of the treatment planning and delivery process. It is assumed, and hoped, that when all the components are correct that the end result is correct. Yet without a feedback mechanism for the entire treatment planning and delivery process, failure to detect a problem with any component or underlying concept wilt most likely go unnoticed.
The only feedback mechanisms commonly employed consist of making a surface measurement on the patient's skin. This surface measurement can be related to a predicted dose value. However, the measurement at one or a few points does not demonstrate how the effects of all the treatment beams are adding up, nor show the dose to the target volume or critical structures. Errors can still exist at other positions within the radiation field that will not be detected, such as performing the point measurement on the central ray but the wedge 26 was reversed in position. Making measurements inside the patient is generally limited to a few points if there is a cavity available and is an invasive procedure.
Prior art includes a method for verifying the dose delivered to the patient by measuring each beam after it transverses the patient's body. Each measured ray is then back traced through the patient's body to predict the intensity of the beam before entering the patient's body. The beam intensity before entering the body is used with conventional calculation methods to compute the dose that the patient receives. However, the measured intensity of each exit ray cannot be used directly as radiation will also reach the same detector that has scattered from within the patient's body and this scatter dose contributes to the measured signal. This scatter dose can be significant and therefore needs to be subtracted from the measured exit dose prior to back tracing through the patient, but knowledge of the beam beforehand is needed to compute the amount of scattered dose. But it is this same knowledge which was to be derived from the exit dose. So the radiation beam needs to be known in order to subtract the scatter component in order to know the radiation beam. Such problems are difficult although not impossible to solve. Further, the computation of the scatter component in itself, given a known beam, is a difficult problem. These difficulties combined with the back ray tracing through the patient are very likely to result in large uncertainties in the final result that will greatly reduce the reliability and therefore usefulness of the final result for verification purposes. Systems which intend to compute the dose distribution from measured exit dose have yet to demonstrate an accuracy to be of sufficient use.
Object and Advantages
In this invention an advantage is obtained by measuring each beam prior to its entering the patient rather than to attempt to derive if from an exit measurement after the beam has transversed the patient. In our invented process such a measurement can be achieved directly with present methods and therefore be accomplished more accurately, as uncertainties introduced by scatter and attenuation in the patient, if measuring exit dose, are not introduced. The measured beam intensity is then used with conventional and known computational methods to compute the dose to the patient achieving an accuracy near that of the original treatment plan.